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Old 14th Jan 2020, 10:12 pm   #1
AD360 Rob
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Default Philips NG5158 no tone control

Evening all,
I've had this neat little mono valved player knocking around for a while now, It's been in and out of the "roundtuit" pile a couple of times, First time out I serviced the deck (A BSR UA25 variant with tubular arm, XS5M cart and Philips branding) and gave the amp a quick look at, powered it up and it worked. . sort of, Output was OK volume wise but no tone control operation. Put it bacK to one side for a few months, tried it again, very low output and HUM, This was traced to tonearm wire broken at cartridge terminal, re soldered connector, tested, back to no tone control operation put back on the pile.
Got it back on the bench today for a proper investigation thinking perhaps the tone control pot had an O/C wiper but no. It turns out that the entire first stage (triode section of the UCL82) has been bypassed and the output from the vol control connected to the pentode section grid (Pin 3) The tone control works on the triode section in this player.
Why would someone do that? they even removed the coupling cap and resistor too but left all the other components connected to the triode section.

I'm going to reinstate the first stage and tone control and see what happens.
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Old 15th Jan 2020, 9:40 am   #2
Edward Huggins
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Default Re: Philips NG5158 no tone control

What a nasty bodge. Yes, I would definately restore the first stage to its original specification. That BSR X5M will not have a sufficient enough output to fully load the pentode secion of a UCL82. It should be ideal when the amp is re-instated with that rather neat NFB tone control circuit.
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Old 15th Jan 2020, 3:34 pm   #3
AD360 Rob
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Default Re: Philips NG5158 no tone control

Indeed a nasty bodge but I can't for the life of me figure out why it was done.

Anyway, stage reinstated and all other components checked. I did find the anode load of the ucl82 triode section had been replaced with a 180k so a 220k was fitted as per cct diagram. Powered up and wow, what a difference! That first stage really brings the player to life, it always sounded rather harsh before, now it's much more controlled and the tone control works beautifully plus its loud now (was OK before but now much better).
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Old 15th Jan 2020, 9:42 pm   #4
AC/HL
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Default Re: Philips NG5158 no tone control

Perhaps someone had read about the idea that tone controls were not necessary to appreciate what the Recording Engineer intended?
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Old 16th Jan 2020, 8:10 am   #5
AD360 Rob
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Default Re: Philips NG5158 no tone control

That is indeed a possibility but I would have thought there are better ways of defeating the tone control especially on a simple 2 stage amplifier
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